A switch mode power supply, which is also known as a switching power supply, a switching regulator, a DC to DC converter, etc., is commonly used in applications requiring high efficiency, minimal heat dissipation, multiple voltages and higher powers, for example, over 10 watts. A switching power supply controls output current by pulse width modulating a power switch and has an advantage of being very efficient and highly responsive to the current requirements of the load. The responsiveness of a switching power supply is constantly being improved by increasing the rate or frequency at which the power switch is operated. Newer designs operate the power switch in the megahertz range. A switching power supply uses an inductor-capacitor circuit that stores energy when the power switch is turned on, and the inductor-capacitor circuit supplies energy to the load when the power switch is turned off.
In order to have a fast response time, the capacitor in the inductorcapacitor circuit must have a very low inductance, which is an inherent characteristic of ceramic capacitors. However, as the operating frequency of the power switch increases and pulse rise times decrease, the inductance of the capacitor must be reduced even further. In the last several years, commercially available stacked ceramic capacitors have an inductance substantially less than 100 picoHenry (“pH”), for example, about 25 pH. The reduced inductance is achieved in several ways, for example, by optimizing the aspect ratio of the capacitor chip size, providing interdigitated multiple terminations, etc.
Although performance is improved, the interdigitated arrangement of capacitor terminations has several disadvantages. First, as shown in FIG. 9, an interdigitated stacked ceramic capacitor 90 comprised of stacks of capacitors 89 has first A terminal strips 91 that are to be connected between capacitor terminals in one capacitor stack and a first, electrically conductive A track or trace 93 on the substrate 94. The interdigitated capacitor 90 has second B terminal strips 92 that are to be connected between capacitor terminals in an adjacent capacitor stack and a second electrically conductive B track or trace 95 on the substrate 94. The arrangement of alternating terminal strips 91, 92 can be on either one side 98 of the interdigitated capacitor 90 or on both sides 98, 99. The alternating arrangement of the A and B terminal strips 91, 92 means that adjacent stacks of capacitors connected to the terminals 91, 92 will have opposite current flows. As can be seen in FIG. 9, one set of the terminal strips 91,92, for example, the A terminal strips 91, can be easily connected to a track 93 of a PC board 94. However, running the conductive track 93 to all of the A terminal strips 91 leaves little space for running a conductive track to all of the B terminal strips 92. The B terminal strips 92 are connected to tracks 96, and often vias 97 are used to connect the B terminal strips 92 to a common track (not shown) on an opposite side of the PC board 94. As the number of interdigitated capacitors increases, the complexity of the PC board layout goes up dramatically and presents a significant PC board design challenge. Therefore, the use of interdigitated capacitors requires that the PC board be designed from scratch specifically for such capacitors.
A second disadvantage is that an interdigitated capacitor cannot be used as plug-in replacement for a noninterdigitated capacitor. A noninterdigitated capacitor has all current flows in the same direction. Therefore, all of the terminal strips on one side of the capacitor are connectable to a PC board track beneath one side of the capacitor, and all terminal strips on an opposite side of the noninterdigitated capacitor are connected to a PC board track beneath an opposite side of the noninterdigitated capacitor. Thus, the PC board layout is substantially simpler than that required for interdigitated capacitors and is incompatible with the requirement of opposite current flows in adjacent capacitors in interdigitated capacitors. Since, interdigitated capacitors cannot be used as a “drop-in” or direct replacement for noninterdigitated capacitors, interdigitated capacitors cannot be used to improve the performance of PC boards designed for noninterdigitated capacitors.
Therefore, there is a need to provide a capability of using interdigitated capacitors, which does not have the disadvantages discussed above.